A computer simulation model for predicting nighttime dissolved oxygen (DO) loss was developed for manured and unmanured tilapia ponds. The model incorporated data from the literature on O2diffusion, respiration by the planktonic community, fish, and benthic organisms, and O2consumption by organic manure. The average absolute deviation between calculated and measured DO concentrations was 0.49 mg/liter for unmanured ponds and 0.38 mg/liter for manured ponds. The contribution of plankton respiration to the total DO loss averaged 75% and 80% in unmanured and manured ponds, respectively. The utility of the model for predicting DO loss over a wide range of plankton densities, fish standing crops, and percentage O2saturation values at dusk was demonstrated. The nighttime DO model should have universal applicability to other fish cultures provided a reliable method for estimating oxygen consumption by the cultured organism(s) is available.